In addition, you probably want your microcontroller to be performing tasks other than LED PWM brightness control, so there has to be some spare execution time between interrupts to do all of the other more general processing tasks.
#Led brightness control code#
Another limiting factor is the code execution, the microcontroller must not only time the ‘interrupt’ which causes the pulse generation, but also run the code which controls the LED output, which must complete before the next interrupt is called. The faster the microcontroller, the smaller durations it can time. However, since the duty-cycle is ‘fixed’ at 50Hz more resolution requires finer timing from the microcontroller. The higher our PWM resolution is, the more levels of ‘brightness’ we can display. The accuracy with which we can control the duty-cycle is known as the ‘PWM resolution’. Since we are using such small time measurements it’s more useful to use microseconds (there are 1,000,000 microseconds in a second), this gives us a cycle duration of 20,000 microseconds which is 50 cycles per second or 50Hz.ĭuring the 20,000 microseconds we have to turn the LED either on or off depending on the required duty-cycle so, for example, a 75% duty-cycle requires the pulse to be on for 15,000 microseconds and then off for 5,000 microseconds. So at 50Hz our cycle is 1 second divided by 50 cycles, which is 0.02 seconds. The ‘cycle’ itself is measured (usually) in Hertz which gives us the cycles-per-second. The duty-cycle refers to the total amount of time a pulse is ‘on’ over the duration of the cycle, so at 50% brightness the duty-cycle of the LED is 50%. When using PWM there are certain terms which you will come across again and again. However, for the purposes of this article, we will use a minimum speed of 50Hz, or 50 times per second (the same speed as used by European televisions). The minimum speed of an LED oscillating which can be seen by the human eye varies from person to person. This technique is how televisions display a seemingly moving picture which is actually made up of a number of different still frames displayed one after the other very rapidly. The eye’s inability to see rapid oscillations of light is caused by our ‘persistence of vision’ which means, in very simple terms, we see the light as on even after it has turned off. When using PWM it’s important to consider how slowly we can ‘flash’ the LED so that the viewer does not perceive the oscillation.
By varying (or ‘modulating’) the pulsing width we can effectively control the light output from the LED, hence the term PWM or Pulse Width Modulation. The pulsing width (in this case 50%) is the important factor here. The important factor here is the ‘duration’, if we turn the light on and off too slowly the viewer will see the flashing of the LED not a constant light output which appears dimmer. If, over a short duration of time, we turn the LED on for 50% and off for 50%, the LED will appear half as bright since the total light output over the time duration is only half as much as 100% on. PWM provides the ability to ‘simulate’ varying levels of power by oscillating the output from the microcontroller.
Although it is possible to supply a varying power from a microcontroller (using a Digital to Analogue Converter (DAC)) this usually requires an additional chip. Microcontrollers are digital, meaning they only have two ‘power’ states, on and off. To control the brightness of an LED you can vary the power which is sent to the LED, for example using a potentiometer (variable resistor), the more power the LED receives the brighter it is, the less power it receives the dimmer it is.
#Led brightness control full#
You can watch the full video on LBRY: Introduction This tutorial is primarily designed as a video tutorial. Some microcontrollers include PWM modules which perform all of the hard work for you however this article focuses on the more universal (and scalable) technique of using interrupts. This article will focus on some of the more specific details of the PIC18F range of microcontrollers however the techniques and principles are the same for all other microcontroller products. In this tutorial we will cover the basic principles behind PWM and how it can be used for LED brightness control including fading out LEDs rather than just turning them on and off. It has many applications, although one of the most popular amongst hobbyists is controlling the brightness of LEDs. Pulse Width Modulation or PWM is a term you hear a lot if you are interested in controlling power output using a microcontroller.
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